Directing droplets using microstructured surfaces

Systematic variation of microscale structures has been employed to create a rough superhydrophobic surface with a contact angle gradient. Droplets are propelled down these gradients, overcoming contact angle hysteresis using energy supplied by mechanical vibration. The rough hydrophobic surfaces have been designed to maintain air traps beneath the droplet by stabilizing its Fakir state. Dimensions and spacing of the microfabricated pillars in silicon control the solid-liquid contact area and are varied to create a gradient in the apparent contact angle. This work introduces the solid-liquid contact area fraction as a new control variable in any scheme of manipulating droplets, presenting theory, fabricated structures, and experimental results that validate the approach.     

Contact instability in adhesion and debonding of thin elastic films

Based on experiments and 3D simulations, we show that a soft elastic film during adhesion and debonding from a rigid flat surface undergoes morphological transitions to pillars, labyrinths, and cavities, all of which have the same lateral pattern length scale, lambda close to lambda/H approximately 3 for thick films, H > 1 microm . The linear stability analysis and experiments show a new thin film regime where lambda/H approximately equal to 3 + 2pi(lambda/3 muH)1/4 (gamma is surface tension, mu is shear modulus) because of a significant surface energy penalty (for example, lambda/H approximately equal to 6 for H = 200 nm; mu = 1 MPa).     

Oxidation of propane to acrylic acid and acetic acid over alkaline earth-doped Mo-V-Sb-Ox catalysts

Alkaline earth metal (Mg, Ca, Sr and Ba)-doped Mo-V-Sb-Ox catalysts, prepared by a dry-up method, have been investigated for their catalytic performance in the oxidation of propane under different reaction conditions. The catalysts have been characterized by N2 adsorption-desorption, temperature-programmed desorption (TPD) of NH3, SEM and XRD. Influence of water vapor on the catalytic performance, particularly on the selectivities to acetic acid and acrylic acid, has also been studied. The selectivity to acrylic acid was improved significantly by the doping of alkaline earth metals to Mo-V-Sb-Ox catalysts. The surface acidic sites of the catalyst decreased with the doping of the catalyst with alkaline earth metals, which ultimately was found to be beneficial for obtaining high selectivity to acrylic acid. The catalytic activity and product selectivities were found to be influenced by the reaction temperature, C3H8/O2 ratio and space velocity. A significant improvement in the selectivity to acrylic acid has also been observed by the addition of water vapor in the feed of propane and oxygen in the oxidation of propane.     

Quantum chemical investigation of the reaction of O(3 P 2) with certain hydrocarbon radicals

The reaction of ground-state atomic oxygen [O(³ P ₂)] with methyl, ethyl, n-propyl and isopropyl radicals has been studied using the density functional method and the complete basis set model. The energies of the reactants, products, reaction intermediates and various transition states as well as the reaction enthalpies have been computed. The possible product channels and the reaction pathways are identified in each case. In the case of methyl radical the minimum energy reaction pathway leads to the products CO + H₂ + H. In the case of ethyl radical the most facile pathway leads to the products, methanal + CH₃ radical. For propyl radical (n- and iso-), the minimum energy reaction pathways would lead to the channel containing ethanal + methyl radical.     

Pattern formation and dewetting in thin films of liquids showing complete macroscale wetting: from "pancakes"to "swiss cheese"

Based on the complete 3D numerical solutions of the nonlinear thin film equation, we address the problems of surface instability, dynamics, morphological diversity and evolution in unstable thin films of the liquids that display complete macroscale wetting. The twin constraints of complete macroscale wettability and nanoscale instability produce a variety of microscopic morphological phases approximating sharp crystal surfaces with flat tops resembling a mesa or a micro "pancake" or a slice of Swiss cheese. While the maximum thickness of flat regions is found to be independent of the initial film thickness, the precise lateral morphology of microdomains formed depends on the film thickness. As the film thickness is increased, the initial pathway of evolution changes from the formation of small spherical droplets, to long mesas (parapets) and islands, to circular holes, all of which eventually resolve by ripening into a collection of round pancakes at equilibrium. However, beyond a certain transition thickness, a novel metastable honeycombed morphology, resembling a membrane or a slice of Swiss cheese, is uncovered, which is produced by an abrupt "freezing" of the evolution during hole growth. In contrast, the spinodal dewetting in thin films of partially wettable systems always engenders spherical droplets at equilibrium. The equilibrium dewetted area from simulations, as well as from simple mass balance, is shown to decline linearly with the initial film thickness.     

Instability of the interface between thin fluid films subjected to electric fields

The effect of an externally applied electric field on the stability of the interface between two thin leaky dielectric fluid films of thickness ratio and viscosity ratio ris analyzed using a linear stability analysis in the long-wave limit. A systematic asymptotic expansion is employed in this limit to derive the coupled nonlinear differential equations describing the evolution of the position of the interface between the fluids and the interfacial free charge distribution. The linearized stability of these equations is determined and the effect of the ratio of the conductivities, dielectric constants, thicknesses, and viscosities on the wavenumber of the fastest growing mode, kmax, and the growth rate of the most unstable mode, smax, is examined in detail. Specific configurations considered in previous studies, such as a perfect dielectric-air interface, leaky dielectric-air interface, etc., emerge as limiting cases from the general formulation developed in this paper. Our results show that the viscosity ratio, mur, does not have any significant effect on kmax for the interface between perfect and leaky dielectric fluids. In marked contrast, however, mur is shown to have a significant effect on the interface between two leaky dielectrics. Increasing mur from 0.1 to 10 could decrease kmax up to a factor of 5. In general, our results show that the presence of nonzero conductivity in either one or both of the fluids has a profound influence on the length-scale characteristic of the linear instability: a reduction even by a factor of 1/50 in the length scale can be effected when compared to the interface between two perfect dielectrics. These predictions could have important implications in pattern formation applications in thin fluid films that employ electric fields. The variation of kmax and smax on the thickness ratio, beta, indicates in general that kmaxalpha(beta-apha), and smaxalpha(beta-theta), where the exponents alpha and theta (both >0) are found to depend only on the ratio of conductivities, and are largely independent of other system parameters.     

IBX/Sc(OTf)3-promoted one-pot oxidative conjugate addition of allyltrimethylsilane to Baylis-Hillman adducts

Baylis-Hillman adducts undergo smooth one-pot oxidative Michael addition with allyltrimethylsilane in the presence of 2-iodoxybenzoic acid (IBX)/Sc(OTf)₃ under mild conditions to afford homoallyl β-ketoesters in good yields with high 1,4-selectivity.     

Electric field induced instability and pattern formation in thin liquid films

Electrostatic field induced instability, morphology, and patterning of a thin liquid film confined between two electrodes with an air gap are studied on the basis of nonlinear 3D simulations, both for spatially homogeneous and heterogeneous fields. In addition to the spinodal flow resulting from the variation of field because of local thickness changes, a heterogeneous imposed field also moves the liquid from the regions of low field to high field, thus allowing a more precise control of pattern. Hexagonal packing of liquid columns is observed for a spatially homogeneous electric field, which is in accord with the e-field experiments on thin polymer films (Schaffer et al. Nature 2000, 403, 874). For a large liquid volume fraction in the gap, varphi > or = 0.75, the coalescence of columns causes a phase inversion, leading to the formation of air columns or cylindrical holes trapped in the liquid matrix (air-in-liquid dispersion). Locally ordered aligned patterns are formed by imposing a spatial variation of the electrostatic field by using a topographically patterned electrode. For example, multiple rows/lines of liquid columns are formed near the edge of a step-like heterogeneity of the electrode and annular rings of ordered columns or concentric ripples are formed around a heterogeneous circular patch. Simulations predict that the electrode pattern is replicated in the film only when the pattern periodicity, L(p), exceeds the instability length scale on the basis of the minimum interelectrode separation distance, L(p) > or = lambda(m)-d(min). Thus, the formation of secondary structures can be suppressed by employing an electrode with deep grooves and stronger field gradients, which produces almost ideal templating. The number density of the electric field induced patterns can be altered by tuning the mean film thickness (or the volume fraction of liquid in the gap), periodicity and depth (amplitude) of the grooves on the top electrode, and the applied voltage. The implications are in electrostatic lithography, pattern replication in soft materials, and the design and interpretation of thin film experiments involving electric fields.     

Synthesis and crystal structure of bis(benzimidazolium) tetrahalocobaltate(II)

The crystal structures of benzimidazolium tetrahalocobaltates (HBz)₂[CoX₄] (X = Cl and Br) have been determined. The chloride salt is triclinic, P-1, with a = 7.670(3) Å, b = 8.307(3) Å, c = 15.730(2) Å, and = 87.37(3)°, = 84.99(3)°, = 67.72(2)°. The bromide salt is monoclinic, C2/c, with a = 15.568(2) Å, b = 8.063(3) Å, c = 5.762(2) Å and = 91.36(3)°. The structures of the two salts are closely related. Both the compounds contain isolated tetrahedral CoX₄ ^2– anions and benzimidazolium cations. In the chloride salt, three chloride ions are involved in strong hydrogen bonding while only two bromide ions participate in the bromide salt. The greater deviation from the ideal tetrahedral geometry in CoBr₄ ^2– can be related to the less extensive hydrogen-bonding network compared to the chloro complex.